Batocera.linux is a ready-to-run retro gaming OS for single-board computers (including Raspberry Pi 3). A "32 GB" image typically means an image file preconfigured to fill a 32 GB SD card (or an image you will write to a 32 GB card). This guide explains how to get the correct Batocera image for Raspberry Pi 3, how to write it to a 32 GB card, and safety/compatibility notes.
The beauty of Batocera is portability. If you fill up your 32GB card, you can upgrade to a 128GB card without reinstalling. download batocera 32gb raspberry pi 3 link